Young adult's intimate shower leads to discovery of self-pleasure and lack of romantic affirmation

A young man, just turning into a teenager, has a habit of showering with his mother. He's always been a bit perverted, but he never acted on it. One day, he's caught by his sister in the shower, and she teases him about being a pervert. He brushes it off, but the thought lingers in the back of his mind. Later that day, he decides to jerk off. He's always been a bit of a wanker, but this time it's different. He's consumed by the thought of his sister's teasing, and he gets off to it. He's disappointed in himself, feeling like a pervert. But the next day, his sister catches him again, this time pleasuring himself. Instead of scolding him, she joins in, giving him a blowjob. He's shocked but enjoys it. He's still a bit guilty, but he can't deny his desires.